The
Viking
spacecraft were not designed to look for evidence of fossilized
life on Mars. But the opportunity to examine such evidence
has been available for nearly a century. In 1911 a meteorite
landed in Egypt and reportedly killed a dog. This is the famous
meteorite that fell in the village of Nakhla.

The
Nakhla
meteorite originated on Mars, though this was not recognized
until the mid nineteen-eighties. The unusual chemical
composition of this and two other similar meteorites, from Shergotty
in India and Chassigny
in France, were known about. They contain some volatile
chemicals and some unusual isotopes of oxygen. But
no-one had thought of a Martian origin. Work in the
1980's showed that they were young - around 130 to 180 million
years old (compared to 5.5 billion years old for most rocky meteorites)
- and they were apparently igneous - formed from volcanic lava.
Eventually researchers discovered tiny traces of Martian atmosphere
in the rocks and this determined their true origin.

This
is not by any means unlikely; Mars is covered with meteorite
impact craters some of which are of enormous size - hundreds of
kilometers or miles in diameter. Many impacts, even
small ones, are so severe that rocky material from the planet
can be sprayed into space to become interplanetary meteorites
themselves.

Earth
is the largest of the inner planets and, when the additional mass
of the Moon is added, it forms the largest local gravitational
attractor in the vicinity of Mars. It is inevitable that
some matter ejected from Mars will find its way to Earth.
Earth, though a small target compared with the volume of
local space, is like a gravitational vacuum cleaner that pulls
orbiting objects towards it. What is more, because
the matter is orbiting there is plenty of time and many opportunities
for some matter to be inexorably drawn towards Earth.
Eventually some of the ejected debris of Martian impacts
will land on Earth as meteorites. It has been estimated
by Bret
Gladman's team of researchers, at Cornell University, that
as much as 7.5% of the material thrown off Mars reaches Earth;
and 2.5% does so in under ten million years which is within the
dormant lifespan of some known microbes. If microscopic
rock-living life-forms similar to those found on Earth also existed
on Mars, then it is virtually certain that Martian microbes will
have reached Earth.

The
same is true the other way: Meteorite impacts on Earth could
achieve the same result, though the amount of material transferred
will be considerably smaller. The Earth's gravitational effects
tend to prevent the escape of material, both from the surface and
from the the Earth's locality in space. However, it
is quite possible that Earth rocks containing microbes have traveled
to other planets and moons in the Solar System. Should we
discover life on other Solar System bodies, and find it has similarities
to that on Earth, we may never be able to untangle its true origin.

It
is argued by some experts that in the early Solar System it would
have been much easier for life to evolve on Mars than it would have
been on Earth. Earth underwent a cataclysmic event when
the Moon was formed and this would have extinguished any form of
evolving life. It was also bigger and therefore hotter
and less hospitable than Mars for the survival of life - more like
Venus is today. Earth is a bigger target for interstellar
debris than Mars. Impacts are also faster and more devastating,
because the gravitational acceleration is higher. Overall,
Mars would offer better conditions for budding life at that time
than would Earth.

(Above)
Pancake volcanoes on Venus.The early Earth may have been just as inhospitable
to life

Once
it was established that Martian meteorites could be found, scientists
started to look for them. One place where meteorites
in general are relatively easily found is Antarctica and many have
been collected in recent years from this continent, including some
from Mars.

In
1989 three
British researchers from the Open University and the Natural
History Museum (Collin
Pillinger, Ian Wright and Monica Grady) published a paper
on meteorite EETA79001, which had been found in Antarctica.
It was certainly of Martian origin, but it contained a surprise.
There were organic compounds present, which the team claimed would
have been associated with living organisms on Earth. The publication
went unnoticed by the majority of researchers and the public at
the time.

In
1996 though, NASA researchers held a press conference that
attracted front page headlines across the world. They announced
that they had discovered fossilized life in a Martian
meteorite.

The
meteorite had been discovered by NASA scientists in the Antarctic
in 1984. This meteorite, numbered ALH 84001, was found
to have originated on Mars. Most meteorites made of
rock are found to be more than 4.5 billion years old, having solidified
from molten material before the the Solar System itself formed.
ALH 84001 was different. It was ejected into space some
16 million years ago and contained microscopic cavities filled with
gas identical to the atmosphere of Mars, that were the tell-tale
signs of its origin.

The
NASA team working on the meteorite put forward reasoned arguments.
It was estimated that the life-form in meteorite
ALH 84001 entered the original rock about 3.6 billion years
ago. After being blasted off Mars by an impacting meteorite
and traveling for 16 million years ago through space, it landed
in the Antarctic just 20 thousand years ago. The main
evidence for life came from the discovery
of Polycyclic Aromatic Hydrocarbons (PAH)
molecules within the rock. These were more concentrated
near the center and this suggested that Earthly contamination was
unlikely. PAH molecules are organics
associated with life processes.

Minute
caterpillar-like structures were found deep inside the meteorite.
It was claimed these were fossil
bacteria, for they looked similar to some found on Earth.
However, it was about one-hundredth the size of an Earth bacteria
and other scientists argued that it was simply too small and was
just part of the minerals in the rock. So that
seemed an end to the initial euphoria, but then other teams of biologists
came up with an astounding discovery. Super-minute bacteria,
of the type found in the meteorite, do
exist on Earth. They are now known as nano-bacteria.
So the debate swung back in favor of the life protagonists.

.

The
antagonists then came back with a highly critical report on the way
the initial tests had been carried out and alleged that the
sample had been contaminated on Earth, probably while it lay in the
ice for 20,000 years. This is how the argument now stands, though
one has to wonder how the bacteria got so far inside the meteorite
and then became fossilized, in such adverse environmental conditions
and such a relatively short time.

(Left) The Sojourner Microrover from the Pathfinder
Mission examines a rock called Yogi.

The
Pathfinder
lander, which arrived on Mars in 1997, was not actually looking
for life. It has really raised more questions about the life
debate, by suggesting from the rock spectral analysis that andesitic
rocks exist on Mars (ie rock created at a subduction zone like the
Andes). While this does not demonstrate the existence
of life directly, it does imply that liquid water existed for some
considerable time on the surface and this is a pre-requisite
for carbon-based life.

Other
photographic evidence from Viking
and the more recent Global
Surveyor, strongly suggest the existence of oceanic shore lines
in some parts, notably in the low-lying bottom of the Valles Marineris
- the great canyon that runs across nearly a quarter of the planet.
Again this is circumstantial support for the possibility of life.
NASA, the European
Space Agency, or NASDA
of Japan will almost certainly put a lander into the valley within
the next few years. The results will be interesting to see.

Valles Marineris - Mariner 9 image.

The
primary argument against carbon-based life currently being active
on Mars, or active in the past for any length of time, comes from
the fact that the atmosphere is largely carbon-dioxide.
Almost the first thing that developing living organisms do in the
presence of CO2
is use it! Had life existed in any abundance, then the
CO2 would have been converted into something else - limestone perhaps.
Although limestone has been detected it is thought to be oolitic
- precipitated from water and not from living matter. The
argument does not hold together entirely, though. There are
forms of simple, anaerobic life that make no use of carbon-dioxide,
and in fact the nano-bacteria postulated
for Mars are very much of this nature.

Even
if Mars supports no life now, it may have done so at one time.
Studies suggest that the early Martian environment may have been
similar to that of the early Earth. Both planets show
histories of liquid surface water, warmer temperatures, and cometary
and meteoritic impacts, as well as relatively thick carbon-dioxide
and nitrogen atmospheres and volcanic activity. The elements
needed for chemical evolution may have been present on Mars, even
if other factors were unfavorable to the continuing evolution of
life.

Because
of Earth's geologic activity, unaltered rocks dating back to 3.5
billion years are rare, but on Mars over half the planet dates to
about 3.8 billion years ago. So, studying Mars helps us understand
events that occurred while life on Earth was just beginning and
it allows us to investigate the possibility of extinct Martian life.
Recent evidence
of ancient microbial life on Mars has renewed intense interest in
the planet. A number of missions planned for the next decade
will help satisfy that curiosity.